The Reid vapor pressure (RVP) of gasoline has been utilized by the Environmental Protection Agency as a means of regulating volatile organic compounds emissions by transportation fuels and for controlling the formation of ground level ozone. As these regulations become more stringent and as more ethanol (which has a high vapor pressure) is blended into gasoline, C5 paraffins need to be removed from the gasoline pool. Moreover, the need to remove components may also extend to some C6 paraffins. This may result in refiners being oversupplied with C5 paraffins and possibly C6 paraffins.
Disproportionation reactions offer a possible solution to this problem. The disproportionation of paraffins (e.g., isopentane (iC5)) involves reacting two moles of hydrocarbon to form one mole each of two different products, one having a carbon count greater than the starting material and the other having a carbon count less than the starting material, as shown in FIG. 1. The total number of moles in the system remains the same throughout the process, but the products have different carbon counts from the reactants. Additional secondary disproportionation-type reactions can occur in which two hydrocarbons having different carbon numbers react to form two different hydrocarbons having different carbon numbers from those of the feed where the total number of carbons in the products does not change from the total number in the feed (e.g., pentane and octane reacting to form hexane and heptane).
There are a number of different catalysts that have been shown to produce the desired paraffin disproportionation reaction, including zeolites, sulfated zirconias, AlCl2/SiO2, ionic solids, platinum on chlorided Al2O3/Ga2O3 supports, supported ionic liquids, Pt/W/Al2O3 and HF/TiF4. However, these processes have a number of disadvantages. The processes using zeolites, sulfated zirconias, AlCl2/SiO2, ionic solids, and platinum on Al2O3/Ga2O3 supports require elevated temperatures (e.g., 120-450° C.) to carry out the transformation. The HF/TiF4 system is capable of disproportionation at 51° C., but it utilizes dangerous HF. The supported ionic liquid is active from about 85-125° C. and is composed of the Brønsted acidic trimethylammonium cation. Since the ionic liquid's organic cation is composed of this Brønsted acid, the acid concentration within this catalyst is stoichiometric with respect to the ionic liquid and quite high. Moreover, the supported ionic liquid is deactivated by leaching of the ionic liquid from the support. Additionally, the use of a support increases the cost of the catalyst and may result in a chemical reaction of the support with the acidic ionic liquid over time, as happens when AlCl3 is immobilized on silica.
Isomerization processes have been used to improve the low octane numbers (RON) of light straight run naphtha. Isomerization processes involve reacting one mole of a hydrocarbon (e.g., normal pentane) to form one mole of an isomer of that specific hydrocarbon (e.g., isopentane), as shown in FIG. 2. The total number of moles remains the same throughout this process, and the product has the same number of carbons as the reactant.
Current isomerization processes use chlorided alumina, sulfated zirconia, or zeolites in conjunction with platinum. Process temperatures range from about 120° C. for chlorided alumina up to about 260° C. for zeolite type catalysts. These reactions are run at temperatures which allow the feed to reach equilibrium. At lower temperatures, the equilibrium favors the branched isomers possessing the higher octane number.
Isomerization processes utilizing ionic liquids have been developed. For example, US 2003/019767 describes an isomerization process for a paraffin hydrocarbon feed using an ionic liquid as a catalyst. The ionic liquid is formed from an N-containing heterocyclic and/or N-containing aliphatic organic cation and an inorganic anion derived from metal halides. The examples show a catalyst:hydrocarbon weight ratio of 1:1 or 1.5:1. The hydrocarbon feeds examined were normal pentane, normal heptane, normal octane, and 3-methylhexane.
US 2004/059173 teaches an isomerization process for linear and/or branched paraffin hydrocarbons. The catalyst comprises an ionic liquid. Over 25 wt. % of a cyclic hydrocarbon additive is included. The ionic liquid is formed from an N-containing heterocyclic and/or N-containing aliphatic organic cation and an inorganic anion derived from metal halides. The ionic liquid:hydrocarbon ratio in the examples is fixed at 1:1 volume ratio. Metal salt additives or Brønsted acids can be included. The feed is a mixture of C7 hydrocarbons.
U.S. Pat. No. 7,053,261 discusses isomerization of linear and/or branched paraffin hydrocarbons using an ionic liquid catalyst in combination with a metal salt additive. The ionic liquid is formed from an N-containing heterocyclic and/or N-containing aliphatic organic cation and an inorganic anion derived from metal halides. The ionic liquid:hydrocarbon ratio in the examples is fixed at 1:1 volume ratio. The results of the gas chromatograph on the paraffin phase were not reported. The feed is a mixture of C7 hydrocarbons.
All of these references describe isomerization of the feed; none describes disproportionation reactions. All of the references describe the use of ionic liquids having an acid concentration of at least about 3.0 M. The Brønsted acidic ionic liquid used in US Publication 2003/0109767 was [trimethylammonium][Al2Cl7], which has a molar concentration of HCl that ranges from 3.0-4.1M if the density is in the range of 1.1 to 1.5 g/mL. For US Publications 2004/0059173 and U.S. Pat. No. 7,053,261 the Brønsted acidic ionic liquid used was [trimethylammonium][Al1.8Cl6.4], which has a molar concentration of HCl that ranges from 3.3-4.5 M if the density is in the range of 1.1 to 1.5 g/mL. These estimated densities are within the ranges measured for similar ionic liquids.
None of the references indicate the composition of the product mixture; as a result, it is unclear what was actually formed in the reactions. Assuming that all of the other products were disproportionation products (which is unlikely to be correct as Ibragimov et al. teach that cracking occurs in addition to disproportionation (see below), but it sets an upper limit on the greatest possible conversion, yield, etc. for the disproportionation products). The conversion rates corrected for mass or volume were calculated as follows: using the reported iso-selectivity, the selectivity to other compounds was calculated as (100-iso-selectivity). The % conversion was determined from the reported %-iso yield and % iso-selectivity. The % conversion thus determined was used to determine the reaction rate by the following formula: volume rate=(% conversion/time (h))×(mL HC/mL IL) or as mass rate=(% conversion/time (h))×(g HC/g IL). The % conversion was then used with the computed selectivity to other compounds to set an upper limit on the yield of disproportionation products. The yield of the other compounds and yield of isomers was then calculated using the calculated selectivity to other compounds and the total yield. Since the reaction rate is dependent on the ratio of ionic liquid:hydrocarbon, the rates were corrected according to these ratios.
With respect to US 2003/0109767, the corrected conversion rates for mass were very low. For n-C5, the corrected conversion rate for mass ranged was between 3.5 and 18.2. For n-C7, it ranged from 2.6 to 9.3, for n-C8, it was 3.3, and for 3-methylhexane, it was 4.7. For US 2005/059173, the corrected conversion rates for volume ranged from 0.6 to 47.1 for the C7 mixture. For U.S. Pat. No. 7,053,261, the corrected conversion rates for volume ranged from 5.4 to 371.3 in the presence of an additional metal salt.
Isomerization is also described in “Isomerization of Light Alkanes Catalysed by Ionic Liquids: An Analysis of Process Parameters,” Ibragimov et al., Theoretical Foundations of Chemical Engineering (2013), 47(1), 66-70. The desired reaction is stated to be isomerization, and the main isomerization products from n-hexane are said to isobutane, isopentane, and hexane isomers. However, isobutane and isopentane are not the isomerization products of n-hexane as isomerization has been defined above. In addition, the article discusses the fact that a significant amount of an undesirable disproportionation reaction begins to occur after about 2-3 hrs. The article indicates that the disproportionation reaction dominates when the ratio of catalyst to hydrocarbon ratio is 2:1, and that cracking and disproportionation dominate at 333K. Because cracking is occurring, the number of moles formed is increased. The optimum isomerization temperature was 303K. The maximum volume rate they obtained was 26 at their high mixing speeds (900 rpm or more) at 0.5 hr.
Some processes involve isomerization and then a cracking reaction in which one mole of a hydrocarbon forms two moles of product, each with a lower carbon number than the starting material. In FIG. 3, the products are illustrated as an alkene and an alkane. Additionally, the total number of moles increases throughout the process.
Alkylation processes involving ionic liquids are also known. In alkylation reactions, one mole of an alkane and one mole of an alkene react to form one mole of an alkane having a carbon number equal to the sum of the carbon numbers of the starting alkane and alkene, as shown in FIG. 4. In an alkylation process, the total number of moles in the system is reduced.
There is a need for improved processes for disproportionation and isomerization of hydrocarbons.